I've never seen that type of alpine slide before. I've ridden the common type in my younger days. Saw a few people get injured, and scraped myself up pretty good once. The one I was riding, the cart could come off the track. I did a little looking, and as I figured, the vehicle can not come off the track, and there's a seat belt. Very little risk as far as I can see, and no breaks makes complete sense from a physics stand point. Watching the video, there's a number of spots that are inclined up or flat, and you need the inertia to make it through those areas, more so for a light rider. If you allowed the rider to slow themselves, there would likely be areas where you would have people stuck on the track. That would be very dangerous.
Oh, I'd ride it in a heart beat
You can see the lower rollers that keep it from derailing.